ASTRONOMY 3S9 



inwards. Dione, therefore, is responsible for the clearance of 

 particles from the surface of Saturn to the commencement of 

 the Crepe Ring. On the other hand, Rhea is only effective 

 near the boundary of its zone of clearance, so that the Crepe 

 Ring is undispersed in the weaker part of its field. The bright 

 rings are within the weak part of Titan's field of clearance, and 

 have not yet been dispersed. In course of time the Crepe Ring 

 will be dispersed by Rhea and the bright rings by Titan. 



As regards the outer zone of clearance, it is found that the 

 innermost satellite, Mimas, will cause a clearance from a distance 

 of 20 "-2 up to the satellite itself. The outer edge of ring A is 

 at distance 2o"-oi, so that the theory indicates with consider- 

 able precision the termination of the whole ring. For the other 

 satellites, the inner limits of the corresponding zone fall within 

 the area already cleared by Mimas. 



Of the other zones indicated by the theory, all fall outside 

 the outer edge of the outermost ring, except the Cassini division, 

 which is produced by the action of Mimas. 



This discussion, therefore, gives a rational account of the 

 existence of the Crepe Ring and the two bright rings, separated 

 by the Cassini division, and assigns the limits of distance from 

 Saturn with a fair degree of accuracy. In arriving at these 

 results, no account was taken of the oblateness of Saturn, or of 

 the influence of one ring of particles upon another. The 

 numerical agreement between observation and theory might 

 be still further improved by their inclusion. 



The theory does not account for the numerous fine divisions 

 observed by Lowell in the bright rings, though their existence 

 is not excluded by it. In Goldsborough's discussion, the dis- 

 turbance of the ring of particles by a satellite was examined with 

 a view to determining under what conditions there would be a 

 large departure from a fixed circle. Such departure results in 

 collisions with adjacent rings of particles and the clearance of a 

 ring in the neighbourhood of the original circle. But the orbits in 

 which the departure from the circular form does not become great 

 with increase of time may yet become dynamically unstable, if 

 further small arbitrary displacements are imposed upon them. 

 The possibility of further divisions is therefore not excluded. 



Although, therefore, the theory is in a sense not complete, 

 it carries the discussion of the ring system a considerable stage 

 further than previously. 



METEOROLOGY. By E. V. Newnham. B.Sc, Meteorological Office, 

 London. 



The Quarterly Journal of the Royal Meteorological Society for 

 July 1 92 1 (vol. xlvii, No. 199), contains an important paper by 

 Dr. G. C. Simpson, F.R.S., explaining the manner in which the 



